First, a few specifics on heat treating of steels. Tempering is actually a process of reducing the hardness of steel, it's also referred to as "drawing it back". Fully hardened steels are quite brittle and have a high level of internal stress due to the quenching process that causes it to harden. Air hardening steels, which do not require quenching, will actually fracture into pieces just sitting on a shelf if they are not tempered. Properly tempering a specific alloy is actually a rather exacting process. The steel must first be heated to a specific temperature and then cooled at a specific rate. Bottomline, you realy cannot temper most steels without a special furnace designed for doing this. There are some exceptions, oil hardening steels such as O-1 or O-6 can be tempered in teh field by simply heating it to a specific temperature and then letting it air cool, for that you will need an infared thermometer that reads up to about 1200 degrees minimum, the level of temper is determined by the temperature it's tempered at.
Now, concerning lawn mower blades. I would expect that these are normally heat treated to achieve a hardeness in the range of 40-43 on the Rockwell C scale. This is commonly referred to as a "Low Spring Temper", an area which will resist bending but is not so hard to facture if it's hit too hard. BTW, 4140 steel can only be hardened to a range of 42-44 Rc unless it's carburized, so I would expect that many lawn mower blades are made with 4140 steel. It's also possible that they are made using "half hard" 4140 which has a hardness of 28-32 Rc, while it's bit soft it would be areal cost savings because it wouldn't require any post manufacturing heat treat and it's easily machined using Cobalt or Carbide cutting tools. A normal spring temper is in the range of 48 Rc and is hard enough that the part won't bend, it will either spring back to it's original shape or break.
Now to the original question. I would recomend replacing any mower blade that has been heated and straightened more that twice in any specific area. I would also recomend the purchase of an infared thermometer and it's use. After straightening the steel, heat it to 700 degrees and hold it at that temperature for a minimum of 1/2 hour. This will allow the steel to normalize and reduce the tendancy to fracture. However, it will produce a reduction in the hardness so the blade will dull faster than normal. Personally, I would not try to re-use a mower blade that has been bent, there is a real potential for internal fractures forming within the steel when it's bent. I should also point out that a harder blade will have a greater chance of developing internal fractures due to being bent. If you really must save money by straightening a bent blade, start out by purchasing the cheapest, softest, blades you can find. It might also be a good idea to pre-nomalize the blade by heating it to 700 degrees before you ever use it. You'll have to sharpen them more often but the risk of having a blade come apart will be much lower. Keep in mind, any blade that comes apart can produce a fragment of shrapnel flying at better that 200 fps and it can kill. So be as safe as you can instead of sorry.
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